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Full text of "Handbook Of Chemical Engineering - I"

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CHEMICAL ENGINEERING
Hot Cap Engines.—These have an unjacketed cap or vaporizer into which the liquid fuel is injected. The cap is externally heated for starting. Where the fuel injection is not "timed/7 ignition depends on compression as well as cap temperature and its timing is apt to be uncertain, especially at variable loads. Compressions are generally from 50 Ib. upward. High compressions lead to high
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ZO 30 4-0   50   60   70   60 90   100  110  &0 130   140  150 ,   Per Cent of Rated B.Hp
FIG. 30.—Efficiency curves of internal combustion engines.1
mean effective pressures and are associated with high economies. A compression of 50 Ib. will give a fuel rate of about 1.1 Ib. per brake horsepower-hour: 300 Ib. compression will give about half this fuel rate. Kerosene hot-cap engines ae 250 to 500 r.p.m. use 30 to 75 Ib. compression, averaging 60 Ib. (gage). The Hornsby-Akroyd engine with fuel oil of high grade (about 35°) uses 45 Ib. Tht Franchetti (De La Vergne FH) engine uses nearly 300 Ib., with oils as low as 20°. For an engine of Hornsby-Akroyd type, fractional load fuel rates are about as follows:
Relative fuel rates.
FULL LOAD
1.0
THKEE-
FOURTHS
LOAD
ONE-HALF LOAD
ONE-FOURTH LOAD
1.15-1.35    1.45-1.70      1.90-2.30
The low-pressure, engines of this class are used mainly with kerosene or solar oil of about 35°. Heavier oils must be more finely atomized than is possible by pump injection against low pressures. With high pressures, an air blast is used to deliver the oil. This implies an air pressure about twice the maximum existing in the cylinder, often 1,000 Ib. or more. The air compressor must be multi-stage with ample cooling, and air storage tanks are necessary for the supply of fuel when starting. Low-grade oils will require heating in order that they may flow readily.
With fuel injection by pump, the engine may be governed either by varying the pump stroke or by by passing a portion of its discharge back to the supply tank.
Diesel Engines.—In the Diesel engine, air only is compressed to about 500 Ib. pressure, attaining a temperature which is sufficient to ignite the incoming fuel The fuel is sprayed in by compressed air at a controlled rate. The maximum pressure rarely exceeds 550 Ib. At normal load, oil delivery continues for about one-tenth the stroke, giving a mean effective pressure around 100 Ib., and a fuel rate of about 0.5 Ib. per brake horsepower-hour. Theoretically, the fuel rate should improve at light loads. Actually (chiefly on account of friction losses) it increases like that of any other engine, though not as rapidly: three results being as follows:
1 Curve 6, Swiderski, single-cylinder alcohol;  Curve 7, Dentz, single-cylinder alcohol; Curve 11, B4nki, gasoline.   (Cf. Marks, Mechanical Engineers' Handbook, 1st. ed., p. 1029.)